This invention relates to a power supply circuit for charging a battery comprising, a first series arrangement of a primary winding of a transformer, a first transistor, a first resistor and a second series arrangement of a secondary winding of the transformer and a first rectifier diode. The second series arrangement is provided with connection terminals for connection to the battery. A positive feedback circuit including a first capacitor is connected between the junction of the secondary winding and the first rectifier diode and the base of the first transistor. A second transistor is coupled to the first resistor for turning off the first transistor. A switching amplifier is arranged between the connection terminals of the battery for turning on the second transistor above a first value of the battery voltage and for turning off the second transistor below a second value of the battery voltage that is below the first value. The switching amplifier has a first input, a second input and an output, said first input being coupled to a tap on a voltage divider which is arranged between the connection terminals of the battery by means of a switch during the periods when the first transistor is turned off. The second input is coupled to the connection terminal of the battery facing the first rectifier diode, and said ouput is coupled to the base of the second transistor.
A circuit of this type can be used for charging a battery from different input voltages. The input voltage may be both a rectified alternating voltage and a direct voltage. A circuit of this type is particularly suitable for use in a shaver in which the circuit is used for charging the battery and/or as the power supply of the motor.
A power supply circuit of the type mentioned in the opening paragraph is used in the Philips' shaver which is commercially available under type number HP 1335 at the time of filing this Application. In this circuit a current flows through the primary winding during the so-called forward interval resulting in energy being stored in the transformer. At a given value of the primary current the second transistor is turned on by the voltage across the first resistor and consequently the first transistor is turned off so that the primary current is interrupted. The energy stored is then applied in the form of a charge current to the battery via the secondary winding and the diode during the so-called flyback interval. After the flyback interval the next forward interval is initiated by the positive feedback between the secondary winding and the base of the first transistor. In this manner the battery can be charged relatively rapidly with a comparatively large current.
To avoid damage to the battery due to overcharge, the power supply circuit is provided with a switching amplifier for turning off the power supply circuit above a first value of the battery voltage and subsequently releasing the circuit at the instant when the battery voltage has decreased to below a second value determined by the hysteresis of the switching amplifier. Thus there is a change-over from rapid charge to trickle charge after the first value has been exceeded for the first time.
A circuit of this type is also known from European Patent Application No. 95 072. In these known power supply circuits the switching amplifier is connected to the connection terminals of the battery throughout the flyback interval. During this flyback interval the battery voltage may exceed the first value of the switching amplifier due to the voltage drop caused by the charge current across the internal resistance of the battery, so that the second transistor is turned off. To eliminate the influence of the internal resistance of the battery on the measurement of the battery voltage, the battery voltage determines, only at the end of the flyback interval, i.e. at the instant when the charge current becomes zero, whether the second transistor remains actually turned on and hence whether the next forward interval is blocked.
However, the following problem occurs in these circuits. When the second transistor is turned on during the flyback interval by a battery voltage above the first value of the switching amplifier, this second transistor is to be turned off again if the battery voltage is not above this value at the end of the flyback interval. It takes some time to turn off the second transistor as a result of the presence of capacitive charges. This delays the initiation of the next forward interval, which disturbs the satisfactory operation of the circuit.